The present invention relates to a storage subsystem storing data for a computer to refer to and to update, and more particularly to the method of duplicating the data held by a storage subsystem and a data duplicating system.
Companies and financial institutions are known to have both computer centers and backup computer centers preparing for the disasters of earthquake, etc. In systems which have backup facilities, the distances between the main computer centers and the remote computer centers are from a hundred kilometers to several hundreds kilometers apart geometrically, and the data are held in disk subsystems installed in the main centers and the remote centers with the duplicating. Systems having so called remote copy functions, wherein writing data generated at the disk subsystem of the main center are transferred to the disk subsystems of the remote center and the transferred same data are written into the disk subsystems of the remote center, have been already realized. The remote copy functions are classified into a synchronous type and an asynchronous type.
The synchronous type means a processing procedure that, when an update (writing) instruction is issued from the host computer (host unit) in the main center to the disk subsystem, the completion of the update processing is reported to the host unit of the main center after the instructed update (writing) to the disk subsystem of the remote center is completed if the instructed object is the object of the remote copy function. In this case, time delay is generated by transmission time caused by the performance of data transmission lines connecting between the main center and the remote center according to geometrical distances.
On the contrary, the asynchronous type means a processing procedure that, when an update (writing) instruction is issued from the host unit in the main center to the disk subsystem, the completion of update processing is reported to the host unit immediately after the completion of update processing at the disk subsystem in the main center, if the object of the instruction is the object of the remote copy function, and the data update (reflection) to the disk subsystem of the remote center is executed asynchronously with the processing regarding to the main center. With the asynchronous type, the data update is completed within the processing time required in the main center and the time delay caused by the data storing into the remote center is not generated. Therefore, when the highest priority is to avoid the influence of the transmission time to the operation of the main center in the remote copy between the distant disk subsystems, the asynchronous type remote copy will be more preferable than the synchronous type remote copy.
With the asynchronous remote copy, the data of the disk subsystem of the remote center is not always the same with the data on the side of the main center. The data that has not been reflected on the side of the remote center may be lost when the main center loses the function by disasters. However, the accessing performance of the disk subsystem on the side of the main center can be maintained at similar level with that of the case where the remote copy function is not applied.
With such prior arts, there have been kinds of problems for achieving the remote copy function with the intervention of the host unit.
In the execution of the remote copy, an independent communication link connects between the disk subsystem of the main center and the disk subsystem of the remote center. That is, in the execution of the remote copy between the two or more disk subsystems of the main center and the two or more disk subsystems of the remote center, the configuration comprises two or more pairs of the disk subsystems connected by the independent communication links. In performing the backup of the main center having two or more disk subsystems by the remote center, there is a problem of maintaining the update sequence of the data among the two or more disk subsystems, that is “the maintenance of the consistency”. In the asynchronous remote copy, it is unavoidable that the reflection of the updated data to the remote center delays from the time of actual update processing at the main center. However, the sequence of the update must be in conformity with that of the main center. At least, at the moment a user requests data at the remote center, the data maintained with the consistency must be stored at the remote center.
Generally, a database comprises a database main body, kinds of log information that record the history of data updates, and control information, and there are many cases of system design where each of the database main body, the kinds of log information, and the control information are stored in different disk subsystems for the security reason. As there is a relativity among the database main body, the kinds of log information and the control information, these log information and control information are supplemented or updated at the data update processing giving to that of the database main body for maintaining the consistency of the system. These series of update are executed in sequence with time intervals of the order of several microseconds at the shortest. If the sequence of the update is confused, the consistency of the information regarding to the sequence of the update may be lost leading to the total destruction of the database.
For example, if the update of the log information, etc. is executed after the update of the database at the main center, there is a possibility of the updated log information, etc. arriving at the remote center before the arrival of the updated information of the database main body depending on the circumstance of the above described communication links comprising the remote copy system. Therefore, there is a hidden possibility of generating the situation where the supplementing or updating of the log information, etc. at the remote center is executed in advance of that of the data base main body at the remote center. If the main center suffers from disasters at the state of logical unconformity where only the log information, etc. are supplemented or updated but the database main body relating to the log information is not updated, the database itself of the remote center cannot be of use. Therefore, there is a problem that the update of the data at the remote center must be executed with the same sequence of the sequence of the update of the data at the main center.
In case of realizing an asynchronous remote copy in the circumstance of both main center and remote center with two or more disk subsystems, such technologies are known that the host unit at the main center gives information related to the update sequence as a time stamp to the data when the host unit at the main center instructs the update of the data to the disk subsystem, and the host unit of the remote center executes the reflection processing of the update of the data to the disk subsystem of the remote center based on such time information. For example, the Japanese patent unexamined publication 6-290125 (U.S. Pat. No. 5,446,871 discloses such technology. The Japanese patent unexamined publication 6-290125 discloses the realization of a remote copy function with the intervention of a host unit. Specifically, by the cooperative operation of the operating system and the disk subsystems of the host unit on the side of the man center and the data mover software and the disk subsystems of the host unit on the side of the remote center, the issue and the transmission of the update sequence information and the reflective processing of the update data based upon the update sequence information are realized.